You can`t cluster only 3 planets.
If you`re terran, miner, guardian or marauder, you need to research Cluster Level 1, then you will be able to cluster 5 normal planets into one new colony. It will work like if it was a normal planet but with all the other 5 planets stats joined. That means a C1 will add 5 planets to your planet counter, but if you lose a battle and the it is on your borders you will lose the entire C1 as if it was a normal planet. There is also C2, 5 C1s together, and C3, 5 C2s together.
If you`re a viral or collective, you can only cluster 4 colonies at a time.